ES2761606T3 - Method and apparatus for managing circuit-switched calls on user equipment - Google Patents

Abstract

A method in a user equipment comprising: detecting that a "CS" circuit switched call has been requested in the user equipment; determining whether a more recent registration attempt or registration update attempt with a first radio access technology "RAT" network resulted in an abnormal state; Register with a second RAT network (120, 122) when abnormal status is detected even if the tracking area code or location area code of a last successful network registration matches the tracking area code or the location area code in a broadcast message from the second RAT network; and initiate the CS call with the network of the second RAT (120, 122).

Description

DESCRIPTION

Method and apparatus for managing circuit switched calls on user equipment

Disclosure field

The present disclosure is directed generally to circuit switched calls, and more specifically, to the performance of mobile originated circuit switched call.

Background

The Third Generation Partnership Project (3GPP) defines a long-term evolution (LTE) architecture, which provides high data rates, low latency, packet optimization, and better system capacity and coverage. In an LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved B-Nodes (eNBs) and communicates with a plurality of mobile stations, also called user equipment (UEs).

LTE technology only supports packet-based services. However, recovery is also specified for circuit switched (CS) services. CS Retrieval (CSFB) defines a mechanism to use a legacy CS network (for example, the Broadband Code Division Multiple Access Global System for Mobile Communications (GSM) (WCDMA)) to provide voice and domain services traditional CS (for example, voice call, a short message service CS (SMS), Data Unstructured Supplementary Services (USSD), among others). To provide these CS services, a UE can reuse the CS infrastructure when E-UTRAN serves the UE.

PCT Patent Application No. WO2013 / 080098 describes a communication method comprising: detecting, through a user terminal connected to a public land mobile network, PLMN, through a first radio access technology and having requested a circuit switched recovery service procedure, there is no change in network controlled radio access technology related to circuit switched recovery service procedure; and selecting a second radio access technology to be applied based on the stored information, wherein the second radio access technology to be applied is capable of providing circuit switched services and the stored information associates at least one PLMN with at least one radio access technology.

US2010 / 329244 describes a system and method for enabling voice services using a network component in a communication system that includes user equipment. The UE is configured to receive a first message which may be a SIP request. The network component is configured to create a second message, or SIP request, based on the first message. The network component was further configured to subsequently receive a SIP response and to select a subsequent action upon receipt of the SIP response.

PCT Patent Application No. WO2010 / 105222 describes methods to facilitate the acquisition of radio resources when an initial redirection attempt to acquire the necessary resources fails. The disclosed embodiments allow the determination of whether or not a desired service is available on a target set of radio resources. In the event of failure to acquire the service, a preferential search of additional radio resources for the acquisition of the service is carried out. The disclosed concepts are applicable to a variety of technologies, such as UMTS, GSM, 3GPP WCDMA and LTE, 3GPP2 IxRTT and HRPD, WLAN, WiMAX, DSL and Passive Optical Network (PON).

PCT Patent Application No. WO2011 / 020002 describes methods and apparatus for supporting voice solutions for data-centric terminals. An example method includes sending a combined registration request message for at least one evolved packet system service through a first radio access technology, receiving an EPS service registration acceptance message comprising an indicator indicating that circuit switched recovery is supported, and when the user equipment has a voice-centric configuration, it is reselected to a second radio access technology.

When a CSFB-capable UE selects an E-UTRAN, a Location Area Update / Tracking Area Update (TAU / LAU) is initiated in combination with an International Mobile Subscriber Identity Connection (IMSI) procedure. Also, when a CSFB-capable UE camping in an E-UTRAN selects a tracking area that is different from one stored in it, there is a combined TA / LA update. Also, when a timer for periodic TA / LA expires, there is a combined TA / LA update. Such procedures may result in a mobile originated (MO) call failure.

Brief description of the drawings

Current systems and methods will be better understood with reference to the drawings, in which:

Figure 1 is a block diagram of a mobile communication system;

Figure 2 is a data flow diagram showing an example of a circuit switched recovery process;

Fig. 3 a flowchart showing a method according to the present disclosure in which a call originated from mobile CS fails;

FIG. 4 is a flowchart showing an embodiment of the present disclosure in which the UE is re-registered to avoid CS call failure; and

Figure 5 is a block diagram of an example UE capable of implementing the methods and systems of the present disclosure.

Detailed description of the drawings

The present disclosure provides a method of user equipment as claimed in the claims. The present disclosure further provides user equipment as claimed in the claims. The long-term evolution architecture is primarily packet based and this can present a challenge for circuit switched services such as voice or short message (SMS) services. One option to deal with such a limitation in LTE is called circuit switched recovery (CSFB), where a user equipment will recover a network using a second generation (2G) or third generation (3G) radio access technology (RAT) ) for circuit switched calls while camping on the LTE network for packet switched services.

The UE will retrieve the 2G or 3G RAT on either a mobile-originated (MO) call or a mobile-terminated (MT) call.

To accommodate circuit switched recovery, the Mobile Switching Center (MSC) server for the ^2g or 3G RAT communicates with the Mobility Management Entity (MME) for the LTE network.

Circuit switched recovery is provided in 3GPP LTE (TS) 23.272 Technical Specification, "Third Generation Partnership Project; Technical Specification and System Aspects Group Services; System Circuit Switched Recovery (CS) of Evolved Packages (EPS); Stage 2 "; v. 11.5.0, June 2013. In particular, the system architecture is provided in Figure 4.2-1 of 3GPP TS 23.272 and is reproduced herein with respect to Figure 1. Reference is now made to Figure 1, illustrating an embodiment of a mobile communication system including a 2G / 3G RAT, as well as a L ^t E RAT.

In particular, a UE 110 is a multimode UE capable of communicating with both a LTE RAT network and a 2G / 3G RAT network. As seen in Figure 1, UE 110 communicates with one or both of a Universal Mobile Telecommunication System (UMTS) 120 Terrestrial Radio Access Network (UTRAN) or a GSM Edge Radio Access Network (GERAN). ) 122 for circuit switched services. UE 110 further communicates with an evolved UTRAN (E-UTRAN) 130 for LTE services.

Both UTRAN 120 and GERAN 122 communicate with a general radio packet service (GPRS) support node (SGSN) 140 for packet switched services and with the MSC 160 server for circuit switched services.

E-UTRAN communicates with MME 150.

In the CSFB system, the MSC 160 server further communicates with MME 150.

During normal operation, the UE 110 will camp on the LTE system. If a completed call is received on a mobile, then the MSC 160 server will notify MME 150, which will then send a page to UE 110 via E-UTRAN 130 to transition to UTRAN 120 or GE ^r A ⁿ 122. The UE 110 will then transition during the duration of the circuit switched call. At the end of the circuit switched call, the UE then makes the transition back to the LTE system.

Packet switching services can either be suspended for the duration of the call Circuit switched or can be transferred to SGSN 140 , where UE 110 will continue to receive packet data, albeit at a lower rate than that of the LTE system.

For a mobile originated call, UE 110 can transition to UTRAN 120 or GERAN 122 for the duration of the call. Again, packet switched services can either be suspended or transition to SGSN 140 .

The typical process for a mobile originated call (without PS handover for simplicity) is provided in Figure 2. Figure 2 follows Figure 6.3-1 of 3GPP TS 23.272.

In particular, a UE 210 communicates with an LTE eNB 212 , which in turn communicates with MME 214 .

Recovery for UE 210 has a base station subsystem / radio network subsystem (BSS / RNS) 216 which is controlled by MSC 220 . The data in the recovery RAT is controlled through SGSN 222 and the bearers are controlled using the service gateway / packet data network gateway (SGW / PGW) 224 .

For a mobile-originated call-based recovery, UE 210 first provides an Accessless Spectrum Extended Service (NAS) 230 request to eNB 212 , which forwards the message to MME 214 .

In response, the MME provides a UE context modification request with a CS recovery flag, as shown in message 232 . The eNB 212 sends a response message 234 back to the MME 214 .

Depending on the configuration, the MME 214 may also send a message 236 to the MSC 220 indicating that a CM service request message should be received.

As shown in block 240 , MME 214 may request an optional measurement report to determine a target GERAN / UTRAN cell for redirection.

In the embodiment of FIG. 2 , it is assumed that a network supports cell switching between RAT, and therefore eNB 212 can send message 242 to provide cell switching between RAT to a neighboring GERAN cell, for example. The message may optionally contain a Network Assisted Cellular Change (NACC).

The eNB 212 can then send a UE context release request to MME 214 , as shown in message 244 .

The context of the UE is released, as shown in block 246 . The UE then changes the RATs and can perform one or both of a location area update (LAU) and a routing area update (RAU), as shown in block 250 .

The UE then suspends the packet services by sending a suspend message 252 to BSS / RNS 216 which forwards the message to SGSN 222 . The suspension is then coordinated between MME 214 and SGSN 222 , as shown by arrow 254 and the carriers are updated, as shown by arrow 256 . As indicated above, the example in Figure 2 is for no PS transfer.

At this point, UE 210 sends a CM service request to BSS / RNS 216 , as shown by arrow 260, and BSS / RNS 216 sends the circuit switched message with CM service request to MSC 220 , as shown with arrow 262 .

If the service request is rejected with unknown IMSI rejection cause in the visitor location record (VLR), the process continues at block 270 . For example, this can happen if the MSC has changed and the UE did not update its registry. In other cases, after rejection with unknown IMSI cause in VLR, any subsequent mobile ended call may also fail due to unknown registration status of the device.

If the process continues in block 270 , a denial of service message 272 is forwarded from MSC 220 , via BSS / RNS 216 to UE 210, and then a location area update is performed, as shown in block 274 .

After the location area update, or if the service denial is not received, the mobile originated call may occur as shown by arrow 280 .

After the call, a routing area update may occur, as shown in block 290 .

As will be appreciated, the denial of service at block 270 will cause the MO call to fail and may cause a poor user experience. A cause of rejection may be that the Ue and the network elements are in different states, as provided below.

Reference is now made to Figure 3 , where the process begins at block 310 . In block 310 , the UE begins with a successful LAU procedure in a 2G / 3G RAT such as a UTRAN / GERAN that establishes a location area code / public land mobile network (PLMN / LAC) or the UE that is camped successfully on E-UTRAN.

The process then proceeds to block 312 where a check is performed to determine one or more of three factors: 1) whether the UE is configured to transition from a state of having an RRC connection to the UTRAN / GERAN system to a state of having RRC connection with the LTE system. In other words, the UE has an Inter-RAT reselection for E-UTRAN; 2) if the UE selects a non-stored TA; and 3) if the timer for the periodic update of TA / LA has expired. If not, the process is repeated in block 312 and otherwise the process goes to block 314 .

As a result of Inter-RAT reselection at block 312 , a combined TAU / LAU connection procedure with IMSI can be initiated to register the UE. In some cases, the TAU / LAU may fail due to RLF radio link failure, release of the RRC (Radio Resource Control Protocol) connection from the RAT, or any other reason. The result may be abnormal registration termination. In other cases, the combined update of the tracking area (TA) / location area (LA) can cause abnormal termination.

In the example in Figure 3 , it is assumed that abnormal log completion occurred, as shown in block 314 . The UE will detect the abnormal completion of the registration and may, according to one embodiment of the present disclosure, consider the UE to be in an abnormal state with respect to the registration. As used herein, "abnormal state" refers to any state in which the UE and the network are not properly registered with each other, for example, may be due to RLF, RRC connection release, or other causes during the registration process.

As a result of abnormal completion of registration in block 314 , the Mobility Management Status (EMM) of the Evolved Package System (ESP) is set to REGISTERED and the EMM substate is EMM_REGISTRADO_INTENTO_ DE_ACTUALIZACIÓN.

The CSFB call bit can be set to TRUE because the active RAT is E-UTRAN. Therefore, when a CS domain service is requested, the UE will submit to CSFB to establish a CS domain service.

A check is then performed at block 318 for a CS call request from MO (or another CS domain service). If an MO CS call is not made, the process returns to block 314 .

If an MO CS call request is made, since the registration terminated abnormally, a PLMN selection may be initiated at the gateway due to the MO call request. However, in this case, the PLMN / LAC found may be the same as the UE had in the last successful record in block 310 . As a result, no location update starts.

Based on the above, the UE and the network registry may be out of sync as a result of abnormal registration termination in block 314 . The network may have unsubscribed the UE IMSI due to the previous TAU attempt that was not completed. The UE, on the other hand, assumes that it has been successfully registered in the same PLMN / LAC as in block 310 .

The TAU procedure was unsuccessful during Inter-RAT reselection for E-UTRAN, therefore the EMM was not fully registered to replace the UTRAN / GERAN registration information. The EMM may be in the TRY TO UPDATE state and cannot handle the MO call request in block 318 .

As a result of the MO call request, the UE can initiate the call set-up procedure (SERVICE REQUEST CM 260 of FIG. 2 ) at block 320 . The network may reject the CS call attempt (CM SERVICE REJECT, cause 4, unknown IMSI in VLR), as received by the UE in block 322 . As a result of the call request in block 320 and the call failure in block 322 , the UE can initiate a LAU request and re-register successfully, as shown in block 324 . However, in this case the call attempt has already been canceled or has failed, resulting in a bad user experience.

Based on the above, although the release of radio resources during the combined TAU / LAU IMSI connection (or the combined TA / LA update) is triggered by network conditions, according to one embodiment of the present disclosure, the UE can handle the conditions to establish the voice call successfully. In particular, according to the present disclosure, the non-access stratum state machine in the UE can be modified to ensure that if a registration attempt ends abnormally while detecting a voice call attempt, the UE can initiate an attempt to additional registration, even if the tracking area code (TAC) / location area code (LAC) provided in a broadcast channel message is the same as the last successfully registered TAC / LAC.

Reference is now made to Figure 4 . In particular, the process of Figure 4 begins at block 410 with the precondition that the UE starts a successful LAU procedure in a 2G / 3G RAT such as a UTRAN / GERAN establishing a public land mobile / code network. Location Area (PLMN / LAC) or UE is properly camped on E-UTRAN. The process then proceeds to block 412 where a check is performed to determine one or more of three factors: 1) whether the UE is configured to transition from a state of having an RRC connection to the UTRAN / GERAN system to a state of having RRC connection with the LTE system. In other words, the UE has an Inter-RAT reselection for E-UTRAN; 2) if the UE selects a non-stored TA; and 3) if the timer for the periodic update of TA / LA has expired. If not, the process repeats at block 412 and otherwise the process goes to block 414 .

As a result of Inter-RAT reselection at block 412 , a combined TAU / LAU connection procedure with IMSI can be initiated to register the UE. In some cases, the TAU / LAU may fail due to an RLF radio link failure, release of the network RRC (Radio Resource Control Protocol) connection, or any other reason. The result may be abnormal registration termination. In other cases, the combined update of the tracking area (TA) / location area (LA) can cause abnormal termination.

Again, in the example in Figure 4, it is assumed that the abnormal completion of the registration occurred, as shown in block 414 . The UE will detect the abnormal completion of the registration and may, according to one embodiment of the present disclosure, consider the UE to be in an abnormal state with respect to the registration.

As a result of abnormal completion of registration in block 414 , the Mobility Management Status (EMM) of the Evolved Package System (ESP) is set to REGISTERED and the EMM substate is EMM_REGISTRADO_INTENTO_ DE_ACTUALIZACIÓN.

The CSFB call bit can be set to TRUE because the active RAT is E-UTRAN. Therefore, when a CS domain service is requested, the UE will submit to CSFB to establish a CS domain service. Then a check is performed at block 418 for a CS call request from MO (or another CS domain service). If an MO CS call is not made, the process returns to block 314 .

If an MO CS call request is made, the process proceeds from block 418 to block 420 where a registration attempt with a 2G / 3G RAT network is initiated. The attempted registration may be in the form of a LAU. Additionally, a follow_on_request flag can optionally be used to signal a network not to release the radio resource when the LAU process completes. In fact, in the message accepted by LAU, the network will set a corresponding flag (follow_on_proceed) to 1 so that the UE knows that it can now send the CM_SOLICITUD_DE_SERVICIO. If the follow_on_request flag is not set to 1, the network will release the radio resource connection after the LAU has accepted. Therefore, the follow_on_request flag can be used to streamline the registration process when there is a pending CS call.

In one embodiment, a verification can be done after registration at block 420 to ensure success, and if the registration is unsuccessful, the process may end or block 420 may repeat.

From block 420 , the process proceeds to block 422 where the pending call is initiated. In this case, since registration occurs, the CS call set-up is successful in carrying out Figure 4, acknowledging the bad user experience described above with respect to Figure 3.

The above embodiments can be implemented by any UE. An exemplary device is described below with respect to Figure 5 .

The UE 500 is typically a two-way wireless communication device that has voice and / or data communication capabilities. The UE 500 generally has the ability to communicate with other computer systems on the Internet. Depending on the exact functionality provided, the UE may be referred to as a data messaging device, two-way pager, wireless email device, data messaging capable cell phone, wireless Internet device, wireless device, a mobile device, or a device of data communication, as examples.

When the UE 500 is enabled for bidirectional communication, it can incorporate a communication subsystem 511 , which includes both a 512 receiver and a 514 transmitter, as well as associated components such as one or more antenna elements 516 and 518 , local oscillators (LO ) 513 , and a processing module such as a digital signal processor (DSP) 520 . As will be evident to experts in the field of communications, the particular design of the communication subsystem 511 will depend on the communication network in which the device intended to operate is located. The radio frequency front end of the communication subsystem 511 may be any of the embodiments described above.

Network access requirements will also vary based on 519 network type. In some networks, access to the network is associated with a subscriber or user of UE 500 . A UE may require a removable user identity module (RUIM) or a subscriber identity module (SIM) card to operate on a network. The SIM / RUIM 544 interface is normally similar to a card slot in which you can insert and eject a SIM / RUIM card. The SIM / RUIM card may have memory and contain many key 551 settings and other 553 information, such as identification and subscriber related information.

When the required network registration or activation procedures have been completed, the UE 500 can send and receive communication signals through network 519 . As illustrated in FIG. 5 , network 519 can consist of multiple base stations that communicate with the UE. Furthermore, as described above, a multimode UE can communicate with base stations of different networks 519 , for example, a WCDMA network and an LTE network.

The signals received by antenna 516 through communication network 519 are fed into receiver 512 , which can perform such common receiver functions as signal amplification, low frequency conversion, filtering, channel selection, and the like. A / D conversion of a received signal enables more complex communication functions, such as demodulation and decoding, to be performed on the DSP 520 . Similarly, the signals to be transmitted are processed, including modulation and encoding, for example, by DSP 520 and input to transmitter 514 for digital-to-analog conversion, up-frequency conversion, filtering, amplification, and transmission to through communication network 519 through antenna 518 . The DSP 520 not only processes the communication signals, but also provides control of the receiver and transmitter. For example, the gains applied to communication signals in receiver 512 and transmitter 514 can be adaptively controlled through automatic gain control algorithms implemented in DSP 520 .

The UE 500 generally includes a processor 538 that controls the overall operation of the device. Communication functions, including voice and data communications, are performed through the 511 communication subsystem. Processor 538 also interacts with additional device subsystems such as display 522 , flash memory 524 , random access memory (RAM) 526 , auxiliary input / output (I / O) subsystems 528 , serial port 530 , one or more keyboards, or keypads 532 , speaker 534 , microphone 536 , another communication subsystem 540 such as a short-range communications subsystem, and any other device subsystems generally designated as 542. Serial port 530 could include a USB port or other known port by those skilled in the art.

Some of the subsystems shown in Figure 5 perform communication related functions, while other subsystems may provide "resident" or on-device functions. In particular, some subsystems, such as keyboard 532 and display 522 , for example, can be used for both communication related functions, such as entering a text message for transmission over a communication network, and functions device residents, such as a calculator or to-do list.

Software operating system used by the processor 538 may be stored in a persistent store such as flash memory 524, which may instead be a read-only memory (ROM) or similar storage element (not shown). Those skilled in the art will appreciate that the operating system, specific device applications, or parts thereof, may be temporarily loaded into volatile memory such as RAM 526 . The received communication signals can also be stored in RAM 526 .

As shown, flash memory 524 can be segregated into different areas for both 558 computer programs and 550 , 552 , 554 and 556 program data storage. These different types of storage indicate that each program can allocate a portion of 524 flash memory for its own data storage requirements. Processor 538 , in addition to the functions of its operating system, can allow the execution of software applications in the UE. A predetermined set of applications that control basic operations, including at least voice and data communication applications, for example, will typically be installed on the UE 500 during manufacturing. Other applications could be installed later or dynamically.

Applications and software can be stored on any computer-readable storage medium. The computer readable storage medium can be a tangible or transitory / non-transitory medium such as optical (eg, CD, DVD, etc.), magnetic (eg, tape), or other memory known in the art.

A software application may be a Personal Information Manager (PIM) application that has the ability to organize and manage data elements related to the UE user, such as, but not limited to, email, calendar events, voicemails , appointments and homework items. Naturally, one or more memory stores would be available in the UE to facilitate the storage of PIM data elements. Such a PIM application may have the ability to send and receive data items, via the 519 wireless network. Other applications can also be loaded into the UE 500 via network 519 , an auxiliary I / O subsystem 528 , serial port 530 , short-range communications subsystem 540, or any other suitable 542 subsystem, and installed by a user on the RAM 526 or in a non-volatile store (not shown) for execution by processor 538 . Such flexibility in the installation of the application increases the functionality of the device and can provide enhanced functions on the device, communication-related functions, or both. For example, secure communication applications may allow e-commerce functions and other such financial transactions to be performed using the UE 500 .

In a data communication mode, the communication subsystem 511 will process a received signal, such as a text message or a web page download, and input it to processor 538 , which can further process the received signal to send it to the screen 522 , or alternatively for an auxiliary I / O device 528 .

A UE 1500 user can also compose data elements such as email messages, for example, using the 532 keyboard, which can be a full alphanumeric keyboard or a telephone-type numeric keyboard, or a virtual numeric keyboard, among others, along with display 522 and possibly an auxiliary I / O device 528 . Such composite elements can be transmitted through a communication network through the communication subsystem 511 .

For voice communications, the general operation of the UE 500 is similar, except that the received signals would typically be sent to a speaker 534 and the signals for transmission would be generated by a microphone 536 . Alternative voice or audio I / O subsystems, such as a voice message recording subsystem can also be implemented in UE 500 . Although voice or audio signal output is generally achieved primarily through speaker 534 , display 522 can also be used to provide an indication of the identity of the calling party, the duration of a voice call, or other information related to voice calls, for example.

The serial port 530 in Figure 5 can be implemented in a personal digital assistant (PDA) type UE for which synchronization with a user's desktop computer (not shown) may be desirable, but is an optional component of the device . Such a port 530 would allow a user to set preferences through an external device or software application and would expand the capabilities of the UE 500 by providing information or software downloads to the UE 500 other than via a wireless communication network. The alternate download path can be used, for example, to upload an encryption key to the device over a direct connection, and therefore reliable and trustworthy to enable secure device communication. As will be appreciated by those skilled in the art, the 530 serial port can also be used to connect the UE to a computer to act as a modem or for charging purposes.

Other communications subsystems 540 , such as a short-range communications subsystem, is an additional optional component that can provide communication between the UE 500 and different systems or devices, which do not necessarily have to be similar devices. For example, subsystem 540 may include an infrared device and associated circuits and components, or a Bluetooth ™ communication module to provide communication with similarly enabled systems and devices. Subsystem 540 may further include non-cellular communications such as WiFi or WiMAX, or near field communications (NFC). The embodiments described herein are examples of structures, systems, or methods that have elements corresponding to elements of the techniques of this application. This written description may enable those skilled in the art to make and use embodiments having alternative elements that also correspond to the elements of the techniques of this application. The intended scope of the techniques of this application therefore includes other structures, systems, or methods that do not differ from the techniques of this application as described herein, and also includes other structures, systems, or methods with material differences. of the techniques of this application as described in this document.

Claims (20)

1. A method in a user equipment comprising: detecting that a circuit switched call "CS" has been requested on the user equipment; determining whether a registration attempt or a more recent registration update attempt with a first radio access technology "RAT" network resulted in an abnormal state; registering with a network of a second RAT (120, 122) when abnormal status is detected even if the tracking area code or location area code of a last successful network record matches the tracking area code or the location area code in a network broadcast message of the second RAT; and initiate the CS call with the network of the second RAT (120, 122). The method according to claim 1, wherein the most recent attempt to register with the network of the first RAT (130) is in an abnormal state due to a radio link failure or a resource control connection release radio. The method according to claim 2, wherein the abnormal state is based on a cause of rejection indicating that an identifier is unknown in a visitor location record. 4. The method according to any preceding claim, further comprising, after registering the second RAT (120, 122) in the network, verifying if the registration is successful and, if not, ending the method. The method according to any of claims 1-3, which further comprises, after registering the second RAT (120, 122) in the network, verifying if the registration was successful and, if not, re-registering in the network of the second RAT (120, 122). The method according to any preceding claim, wherein the first RAT (130) is an evolved universal terrestrial radio access RAT "E-UTRAN". The method according to any preceding claim, wherein the second RAT (120, 122) is a RAT of a global system for mobile communications "GSM" or an evolved universal terrestrial radio access RAT "UTRAN". The method according to claim 1, wherein the first RAT is a packet switched RAT only. The method according to claim 1, further comprising re-selecting the network from the second RAT for the CS call. The method according to claim 9, wherein the reselection comprises sending a CM service request to the network of the second RAT. The method according to claim 9, wherein reselecting the second RAT network further comprises registering with the second RAT network. 12. The method according to claim 1, wherein the registration is performed in the form of a location area update using a follow_on_request flag. The method according to claim 1, further comprising initiating a "CSFB" circuit switched recovery procedure when it is detected that a CS call has been requested on the user equipment. 14. A user equipment comprising a processor configured to: detecting that a circuit switched call "CS" has been requested on the user equipment; determine whether a more recent registration attempt or registration update attempt with a first radio access technology "RAT" network resulted in an abnormal state; register with a network from a second RAT when abnormal status is detected even if a tracking area code or a location area code from a last successful network record matches the tracking area code or the area code of location in a broadcast message from the second RAT network; and initiate the CS call with the network of the second RAT. 15. The user equipment according to claim 14, wherein the first RAT is a packet switched RAT only. 16. The user equipment according to claim 14, wherein the user equipment is further configured to reselect the second RAT network for the CS call. 17. The user equipment according to claim 16, wherein the reselection comprises sending a Cm service request to the network of the second RAT. 18. The user equipment according to claim 16, wherein reselecting the second RAT to the network further comprises registering with the second RAT. 19. The user equipment according to claim 16, wherein the registration in the network of the second RAT is in the form of a location area update using a follow_on_request flag. 20. The user equipment according to claim 14, wherein the user equipment is further configured to initiate a "CSFB" circuit switched recovery procedure when it detects that a CS call has been requested on the user equipment.